I hoped for a happy ending for your momma cat. While I took the time to share the best advice I've gotten from countless premier veterinary hospitals over the course of 20+ years (many rescues, many hospitals, many states - I always check with many sources before sharing advice - they all concur with the low dose aspirin every 3 days but never any other OTC products), what you do with it - or don't do with it is, of course , truly and entirely up to you. You needn't declare or defend your decision on meds to anyone.
Sadly, it turned out that momma cat's problems were a lot graver than pain alone in any case........ I am sorry to hear that she died. It did sound like she was in dire need of help. To die she had to feel as bad as it's possible to feel. What unfortunately often happens to unspayed female cats who have had multiple litters is that they get one of the reproductive cancers (this is quite rare in spayed cats). Having this litter may have been just enough stress to weaken her immune system and take the fight out of her if she did have cancer. Alternatively it could have simply been a fixable infection, treated early on. My heart goes out to her - I wish she could have the joy of basking in the sun right now. Very sorry it wasn't a happy ending.
JJ